Carrier sense multiple access (CSMA) communications is a method which employs the terminal's channel-sensing capability to schedule transmissions. Each terminal can only attempt transmission when no others are using the medium. The terminals can determine this condition by sensing whether there is signal energy at the frequency bands designated by the system for transmission. A terminal will reschedule their attempts for a, possibly random, later time when this happens or when its transmission is not successful.
An enhancement to CSMA is CSMA with collision detection (CSMA/CD), for which a transmitting terminal also continually senses the carrier and will stop transmission if any other simultaneous transmissions are detected. As most networks' physical (PHY) layers commonly employ signaling schemes that do not facilitate more than one transmissions at a time, i.e., that a transmission occurring in the presence of any others will result into a collision of their signals and none of them will be received correctly, collision detection can shorten the amount of time the channel is occupied with collisions to the least possible. Subsequently, significant improvement in throughput and delay is achieved. Since the signaling schemes of these PHY layers involve a carrier, CSMA is in fact performed by sensing the presence of this carrier; and thus CSMA is also known as carrier sense multiple access in those contexts.
CSMA/CD has since become the protocol for scheduling packet transmissions in the medium access control (MAC) layer of IEEE 802.3, the de facto standard for wired local area networks (LANs), or what is popularly known as Ethernet. In these systems, the channel is the common wire medium—usually a coaxial cable or in recent years perhaps a fiber optic line—connecting each station in the network. By sensing the signal level on the wire, stations become aware of when another station already has started transmission.
CSMA was also chosen as the MAC layers' access methods of the currently widely adopted IEEE 802.11 wireless LAN (WLAN) standards (IEEE 802.11, 802.11a, 802.11b, 802.11g). Nevertheless, collision detection is not considered because it is commonly agreed upon that this can not be easily implemented in the wireless environment. Different from wired mediums, during transmission a wireless terminal's antenna radiates a significant amount of energy which saturates its radio frequency (RF) front-end. And as a result, during this time the terminal can only receive its own RF signal and cannot detect any transmissions from others that are at the transmission's frequency. Such a phenomenon is commonly known as self-interference and is the reason duplex RF communications cannot occur at the same frequency.
Duplex RF communications can in fact be performed if the wireless terminal transmit and receive at different frequencies. For example, the terminal can either employ two antennae for each of these tasks or one antenna with a circulator. The circulator, also known as the duplexer, is a device that allows the RF transmitter and receiver circuitries to share an antenna by passing through the relevant signal to each of them as required. In either case, these channels need to be sufficiently separated so that a band pass filter can attenuate the transmitted signal enough for proper decoding of the received signal.
With the structure for duplex RF communications defined, CD in a wireless environment can be performed by delegating the detection to the receiving terminal. The system will have two channels, one for data transmission and one for transmitting a feedback signal for indicating a collision has occurred. When the receiving terminal detects there is more than one transmission on the data channel, it will indicate this on the feedback channel. A transmitting terminal can find out if there is a collision by listening to the feedback channel during the transmission.
The first receiver-initiated CD method was proposed in 1988 by Wu and Li. In their scheme, called the receiver-initiated busy tone multiple access (RI-BTMA), the system's bandwidth is divided into two channels, one for data transmission and one for the busy tone. When there is transmission detected on the data channel, the receiver will attempt to decode the preamble of the transmission. If this is successful, as usually is when only one terminal transmits, then the receiver will assert the busy-tone signal until the transmission is over. And so instead of carrier-sensing the data channel for radio frequency activities, in the RI-BTMA scheme terminals monitor the busy-tone channel to determine when others have already started transmissions. And for a terminal that is already transmitting, it can find out if it has become involved in a collision by monitoring whether the receiver has initiated the busy-tone channel.
With the terminals performing carrier sensing on the busy-tone channel rather than the data channel, the protocol also solves another problem common in wireless networks: the hidden terminal problem. Namely, this problem addresses the possibility that a wireless terminal can be within transmission range from a receiver but out of range from some other (“hidden”) terminals such that their transmissions are not detected. If there are on-going transmissions from these “hidden” terminals and if it decides to transmit, then collisions will ensue. Since the receiver should be within range of the terminals that are interested in communicating to it, its busy-tone can be heard by these terminals and so the hidden terminal problem is eliminated.
Wu and Li's RI-BTMA actually only partially solves the hidden terminal problem since the receiver does not begin to issue any busy tone until it successfully has decoded the preamble. During the time needed for the receiver to perform this task, the system is vulnerable to incorrect carrier sensing as terminals may detect no busy-tone and wrongly think that no others are currently transmitting. Therefore, if the receiver can assert a busy-tone as soon as it detects any potential data transmission, as in the original non-receiver-initiated BTMA scheme introduced by Tobagi and Kleinrock, then this vulnerable period would no longer exist.
Such enhancement to the RI-BTMA is precisely what Gummalla and Limb suggested in their 2000 paper. In their scheme, named the Wireless Collision Detect (WCD), the busy-tone channel is called the “feedback” channel and the receiver will assert the “carrier detect” signal on it when there is any signal energy detected on the data channel. If the receiver successfully decodes the preamble of this transmission and that this transmission is destined for it, then it will begin asserting instead on the feedback channel the “feedback” signal. If the preamble is not decoded successfully, as will be if there is more than one concurrent transmissions, i.e., a collision has occurred, then the receiver will stop asserting the feedback signal. And similar to RI-BTMA, based on whether this signal is there, the transmitting terminal would know it is involved in a collision, and if so it will terminate its transmission.
The hidden terminal problem actually may not be of concern for some wireless networks. For example, there are networks where terminals are close to each other, like in a wireless personal area network (WPAN). In these cases, just monitoring the data channel for carrier signal energy would be dependable and suffice to perform CSMA. There will also be no need for a busy-tone or a carrier-detect signal that lasts for the transmission's duration. A receiver-initiated feedback or CD signal would still be needed to perform CSMA/CD, though now this signal can be very short, lasting long enough for the transmitting terminals to detect it.